Annealing of arc-induced gratings at high temperatures

Abstract

Long period gratings arc-induced in different types of fibre were annealed at 10008C for 24 h. During the annealing, the spectrum of the gratings written in Ge-doped fibres shifted towards longer wavelengths whereas an opposite shift was observed for gratings written in Ge-free fibres. A discussion on the mechanisms responsible for that behaviour is presented.Introduction: In recent years, optical-fibre-based sensors have become a market of increasing demand, since owing to their intrinsic properties they offer clear advantages over the conventional ones. It is irrefutable that high-temperature sensors are of great importance and, since the appearance of UV-induced fibre Bragg gratings, much effort has been put forward from the scientific community in order to increase their high-temperature resistance. The electric arc technique is among the few techniques able to produce gratings that can withstand temperatures of the order of 10008C. In past years, the subject of arc-induced gratings and their applications was intensively investigated, namely, long period fibre gratings (LPFGs) were inscribed in different types of fibre, their temperature behaviour was investigated up to 12008C and distinct behaviour was observed for Ge-doped and Ge-free fibres [1]. It was also demonstrated that arc-induced gratings can withstand temperatures of 12008C for short periods. Annealing experiments in the temperature range 900 -11008C, concerning gratings arc-induced in the SMF-28 fibre, were also conducted for a few hours [2]. In this Letter, we present results on the effect of annealing on gratings arc-induced in Ge-doped and Ge-free fibres at 10008C for 24 h.

Investigation of the long-term stability of arc-induced gratings heat treated at high temperatures

“…As can be seen, the spectrum moves towards longer wavelengths, the resonance depth corresponding to the LP 11 mode decreases, while the resonance depth of the LP 12 and LP 13 modes first increase and then decrease. This behaviour results mainly from structural relaxation (a change in the fictive temperature, T F ) that affects more strongly the fibre cladding leading to a larger decrease of the refractive index of the cladding compared to the change in the core [2]. Therefore, the resonance condition leads to a red shift of the gratings spectrum.…”

“…In this context, the development of high temperature sensors based on fibre gratings is of great interest. Long-period fibre gratings (LPFGs) produced by the electric arc technique have already demonstrated good stability at high temperatures during 24 h annealing [2]. In this paper, we assess the effect of heat treatment on arc-induced gratings at 1000°C for two weeks.…”

Arc-Induced Long-Period Fiber Gratings at INESC TEC. Part I: Fabrication, Characterization and Mechanisms of Formation

“…Diffusion of the core dopants was one of the first mechanisms proposed for the formation of the gratings [ 37 , 38 ]. However, for the typical fabrication parameters used and considering the most common fiber composition, the diffusion length is too short, resulting in a negligible contribution [ 102 , 116 ]. Another potential mechanism is stress relaxation [ 94 , 117 ].…”

Arc-Induced Long-Period Fiber Gratings at INESC TEC. Part II: Properties and Applications in Optical Communications and Sensing

“…The thermal annealing of several gratings, inscribed in different type of fibers, at high temperatures was also investigated [ 97 ]. Figure 8 shows the wavelength shift during annealing performed at 1000 °C during 24 h. It is interesting to note that gratings inscribed in germanium doped fibers move towards longer wavelengths, while gratings inscribed in germanium free fibers (pure-silica-core and nitrogen doped fibers) the shift is much larger and is precisely in the opposite direction.…”